The search for kinase inhibitors has proven to be a fruitful area for the development of pharmaceutically active substances. As an outcome of extensive drug discovery and development efforts in this area, the first PI3K inhibitor idelalisib (quinazolinone class of compound) has been recently approved by FDA in 2014, as a combination with rituximab for chronic lymphocytic leukaemia (Nature Rev. Drug Discov. 2014, 13, 162-164). Another PI3K inhibitor (pan-PI3K inhibitor) buparlisib is in phase III clinical trial for treatment of breast cancer. Other PI3K inhibitors in clinical studies include BEZ-235 and BKM-120 (Phase II, Novartis). AstraZeneca's AZD-6482, which is a PI3K-β inhibitor, has completed Phase I trials for the treatment of thrombosis. Another quinazolinone-based isoform-specific PI3K-δ inhibitor IC-87114 (Calistoga) has entered Phase I clinical trial. Other PI3K inhibitors in clinical trials include D106669 and D87503 (Phase I, Aeterna Zentaris), GDC-0941 (Phase I, Genentech) and PKI-587 (Phase I, Pfizer). In addition, several other PI3K inhibitors are in early stages of clinical trials.
Although large numbers of kinase inhibitors have received FDA-approval, the target selectivity remains a formidable challenge in drug development because almost all approved kinase inhibitor drugs works by competing with ATP for the ATP binding site of the enzyme. Hence, there is a great need for next-generation kinase inhibitors that work through alternative mechanisms such as allosteric inhibition. While recently approved kinase inhibitor drugs offer benefits for cancer treatment, further advances are required to effect tumor selective cell killing, avoid off-target related toxicities and improve survival rates (Bharate, S. B. et al., Chem. Rev. 2013, 113, 6761). Amongst the four isoforms of phosphoinositide 3-kinases, particularly the α-isoform has been found to be activated by mutation in several cancers; and therefore discovery of α-isoform selective inhibitor is highly important. PI-103 is a multi-targeted PI3K inhibitor for p110α/β/δ/γ with IC50 of 2 nM/3 nM/3 nM/15 nM in cell-free assays, and also inhibits mTOR/DNA-PK with IC50 of 30 nM/23 nM. Thus, it does not show any selectivity towards α-isoform compared with β, γ and δ isoforms. Furthermore, PI-103 has very poor aqueous solubility (5 μg/ml) and it undergoes rapid metabolism (via glucuronidation of phenolic hydroxyl group).
